[Navy Yard shooter Aaron] Alexis also apparently was not a big presence on social networking sites, and sources say so far they cannot connect him in any way to any known threats or terror groups. But the motive for the shootings remains unknown.from Aaron Alexis, alleged Navy Yard shooting gunman, showed "pattern of misconduct" in military
What I've read about the Navy Yard rampage leads me to suspect that it was a case of possession by the Spirits of Darkness, although I would be speculating if I were attempt to assign a motive to them, because there are so many possibilities. Perhaps one will emerge with time. For more on this sort of possession, see Andrea Yates and the Spirits of Darkness.